This invention relates to a control scheme for controlling the locomotion of a legged robot, and more particularly to a control scheme stabilizing any desired, energetically possible movement trajectory at any given locomotion speed within a single step of a running monopod or other legged robot.
Legged locomotion aims to move an object in an intended direction. To achieve this goal, the system must provide enough energy to compensate for losses within the system (e.g. leg damping) and with the environment (e.g. air friction). Thus, legged robots require a control, which keeps the system energy balanced at a desired level, to reach a steady-state movement. However, even with sufficiently controlled system energy, forward locomotion may fail. If a leg is not properly aligned with respect to the ground level (e.g. the ground is higher than expected), the legged system can stumble or fall over. Thus, legged robots require an additional control, which keeps an aimed movement trajectory corresponding to the controlled energy level, and therefore stabilizes the steady-state locomotion.
Running is a special form of legged locomotion incorporating ballistic flight phases to achieve high forward velocities. As of today, the control of running machines is mostly based on (or in part based on) the use of a scheme elaborated by Marc Raibert and collaborators (M. H. Raibert, “Legged robots that balance”, MIT Press, Cambridge, Mass., 1986). This control scheme decomposes the control of running into three parts: the control of hopping height, forward speed, and posture. It therefore mixes the formerly introduced control of system energy and kinematic trajectory.
Although this decomposed control can result in dynamically stable locomotion, it inherits several difficulties. First, as it represents an empirical feedback control, it requires the collection of appropriate feedback gains. Second, as it presets the legs in one flight phase to a fixed orientation, the robot can stumble when encountering lifted ground or obstacles. Third, the system requires several steps to stabilize after a perturbation. Lastly, due to the particular control of forward speed, the locomotion system spends additional amounts of energy when running over rough terrain: a lifted (lowered) ground level of one stance phase decreases (increases) the forward speed of the running machine, which is compensated by acceleration (deceleration) due to the explicit velocity control. This is not necessarily required when the mean perturbation in ground level is zero as for uneven terrain of our every day experience, and therefore reduces the range of mobility for autonomous legged robots.